Non-aqueous electrolyte secondary batteries represented by lithium secondary batteries are extensively used as power sources for small portable terminals, mobile communication apparatus, etc. because these batteries have a high energy density and a high voltage can be taken out thereof. A lithium secondary battery has a positive electrode employing as a main component a positive active material able to release/dope lithium ions with charge/discharge, a negative electrode able to dope/undope lithium ions with charge/discharge, and an electrolyte comprising a lithium salt and a non-aqueous solvent.
Presently known positive active materials for lithium secondary batteries include: lithium-transition metal composite oxides having a layered structure and having a composition represented by LiMO2 (M is a transition metal element); lithium-transition metal composite oxides having a spinel structure and having a composition represented by LiM2O4 (M is a transition metal element); and lithium-transition metal-phosphoric acid compounds having a composition represented by LiMPO4 (M is a transition metal element). Of these, LiCoO2, which is one of the lithium-transition metal composite oxides having a layered structure and having a composition represented by LiMO2 (M is a transition metal element), is in extensive use as a positive active material for lithium secondary batteries especially for portable communication appliances because it has a high energy density.
Various attempts have been proposed in which the surface of an active material is modified with an element of a different kind to improve performance. In patent documents 1 to 4 is described a method in which the surface of an active material is coated with aluminum to thereby improve electron conductivity. However, this method has been insufficient in inhibiting the oxidative decomposition of electrolytes on a positive electrode, although it surely improves the electron conductivity of the particle surface.
In patent document 5 is described a positive-electrode material obtained by forming a metallic conductive layer comprising indium, magnesium, aluminum, barium, strontium, calcium, zinc, tin, bismuth, cerium, or ytterbium on the surface of base particles. However, the deposition of a zero-valent metal on the surface has not always resulted in satisfactory cycle performance. This is presumed to be because the metallic conductive layer shows insufficient conformability to the expansion/contraction of the active-material particles with charge/discharge. Furthermore, for forming the metallic conductive layer on the surface, it is necessary to conduct a treatment in a reducing atmosphere as described in Examples given in that document. The treatment in such an atmosphere has posed a problem that the positive active material undergoes oxygen extraction therefrom, etc. and this is apt to disorder the crystal structure of the active material, resulting in reduced battery performances. Patent document 6 discloses an attempt to heighten heat resistance and electron conductivity by doping a surface part of Li—Mn—Ni—Co composite oxide base particles with a minute amount of an element of a different kind (Al, Mg, Ca, Sr, Y, or Yb) in such a degree as not to disorder the crystal structure. However, these techniques are insufficient for the modification of the active-material surface and failed to sufficiently improve battery performances. Incidentally, no statement is given in patent document 6 on how much the doping with these elements improves battery performances.
It is becoming known that a lithium-nickel-manganese-cobalt composite oxide obtained by displacing part of the nickel in LiNiO2 by manganese and cobalt as other elements (see, for example, patent document 7) not only shows the same charge/discharge capacity as lithium-cobalt oxide and excellent charge/discharge cycle performance and storage performance but also shows better high-temperature stability in the last stage of charge than lithium-cobalt oxide and lithium-nickel oxide. That composite oxide is hence receiving attention as a positive active material which replaces lithium-cobalt oxide.
In patent document 8 is described a lithium ion secondary battery which employs a positive electrode containing a lithium-transition metal composite oxide and a negative electrode containing a carbon material, wherein the lithium-transition metal composite oxide is, for example, a layered lithium-nickel-manganese-cobalt composite oxide having a specific composition, and which is used at an upper-limit voltage of 4.15-4.4 V. However, it has been desired to further improve charge/discharge cycle performance.
[Patent Document 1] JP-A-08-102332
[Patent Document 2] JP-A-09-171813
[Patent Document 3] JP-A-2002-151077
[Patent Document 4] JP-A-2001-256979
[Patent Document 5] JP-A-2000-048820
[Patent Document 6] JP-A-2003-017052
[Patent Document 7] JP-A-2000-133262
[Patent Document 8] JP-A-2003-264006
Lithium secondary batteries have had a problem that the batteries, when allowed to stand in a charged state over long, deteriorate in battery properties such as discharge performance. This problem was conspicuous especially in lithium secondary batteries which had repeatedly undergone charge/discharge many times. The present inventors made investigations concerning a cause of that problem and, as a result, it was found that in lithium secondary batteries deteriorated in properties, the negative electrode employing a carbon material has an operating potential region which has shifted to the higher-potential side. From this finding, the inventors presumed the cause of the property deterioration to be as follows. Due to the potential applied to the positive electrode, the electrolyte present near the positive electrode decomposes to generate decomposition products consisting mainly of carbonic acid radicals. These acid radicals migrate toward the negative-electrode side to thereby form a coating film consisting mainly of carbonic acid radicals on the surface of the negative electrode to increase the negative-electrode impedance. This results in a substantial increase in negative-electrode potential and hence in the shifting of the negative-electrode operating potential region to the higher-potential side, and the positive-electrode operating potential region shifts to the higher-potential side accordingly. Because of this, a higher potential is imposed on the positive electrode and the phenomenon described above is further accelerated, resulting in enhanced deterioration of battery performances.
The invention has been achieved in view of the problem described above. An object thereof is to provide: a positive active material which can inhibit side reactions between the positive electrode and an electrolyte even at a high potential and which, when applied to a battery, can improve charge/discharge cycle performance without impairing battery performances even in storage in a charged state; a process for producing the active material; a positive electrode for lithium secondary batteries which employs the active material; and a lithium secondary battery which has improved charge/discharge cycle performance while retaining intact battery performances even after storage in a charged state. Another object is to provide a lithium secondary battery which can exhibit excellent charge/discharge cycle performance even when used at a high upper-limit voltage.